In a GSM network, information about other available radio-access technologies (RATs) is included within system information messages sent on the Broadcast Control Channel (BCCH). These other RATS may include, for example, radio-access equipment based on Universal Terrestrial Radio-Access Network (UTRAN) technology, which includes Wideband-CDMA (W-CDMA) technology, and equipment based on Evolved Universal Terrestrial Radio-Access Network (E-UTRAN) technology, also referred to as Long-Term Evolution (LTE).
Information regarding other available RATs may be sent in a system information message called a System Information Type 2quater (SI 2quater) message, and is used by the mobiles in reselection, i.e., to find cells in other RATs (UTRAN and/or E-UTRAN) that the mobile stations can move to while in idle mode or in packet transfer mode. The SI 2quater message is described in standards documents promulgated by the 3rd-Generation Partnership Project (3GPP). In particular, version 10.3.0 of 3GPP TS 44.018 defines this message in general terms as follows:                “This message is sent optionally on the BCCH by the network to all mobile stations within the cell giving information on additional measurement and reporting parameters and/or UTRAN neighbour cells and/or E-UTRAN neighbour frequencies and/or CSG neighbour cells. A mobile station with no UTRAN capability should ignore the 3G Neighbour Cell, 3G MEASUREMENT parameter, GPRS—3G MEASUREMENT parameter and 3G Priority parameters descriptions in this message. A mobile station with no E-UTRAN capability should ignore the E-UTRAN parameters description and GPRS E-UTRAN measurement parameters description in this message. Multiple instances of this message may be sent by the network.”The information in a SI 2quater message is encoded in the form information elements called “rest octets,” which are defined in detail in Section 10.5.2.33b of 3GPP TS 44.018, v 10.3.0 (June 2011).        
A single instance of the SI 2quater message includes an SI 2quater Rest Octets information element having a maximum size of 20 octets. However, information providing neighbor cell information for other RATs might easily exceed this available payload space. This means that the information elements carrying information for other RATs might not fit entirely within a single instance of an SI 2quater message, in which case several instances of this message, together forming a single coherent message set, may be required to carry all of the necessary information. In addition to neighbor cell information for other RATs, GSM-specific system information is sent on the BCCH within the set of SI 2quater messages, as is other system information.
Assuming the BCCH Norm channel format is used, a system information message (e.g., an SI 2quater message instance) may be sent once every multi-frame, which consists of 51 radio frames. 3GPP TS 45.002, chapter 6.3.1.3, v 10.1.0 (May 2011) specifies when the different system information messages may be sent. As specified therein, a value for a Transmission Condition (TC) is calculated based on the radio frame number (0 . . . 2 715 647). TC is calculated as follows:TC=(Frame Number DIV 51)MOD 8
This means that a multi-frame number (corresponding to 51 frames) is first calculated, and the multi-frames are then grouped in 8 different groups. 3GPP standards specify that the SI 2quater message may be sent on BCCH Norm when TC is either 4 or 5, depending on what other information is distributed.
An example of how SI 2quater messages and other system information messages may be distributed on BCCH Norm is shown in Table 1 below. SI 2quater messages are labeled SI2q—1 and SI2q—2 in Table 1, while other system information messages are labeled SI1, SI2, SI3, and SI4:
TABLE 1Example of SI 2quater Messages mapped to BCCH NormTC = 0123456701234567SI1SI2SI3SI4SI2q_1SI3SI4SI1SI2SI3SI4SI2q_2SI3SI4
A GSM multi-frame is 3060/13 milliseconds long. Since each SI 2quater message instance can only be sent every eighth 51-frame multiframe, this means that one SI 2quater message can only be sent once every 3060/13*8 milliseconds, or approximately once every 1.88 seconds. For details of the frame structures in GSM, see chapters 5.1 and 5.2 in 3GPP TS 45.001, v. 10.0.0 (March 2011).
A single coherent set of SI 2quater messages may include, for example 3G Measurement data, 3G neighbour cell information, 3G priority information, Serving Cell Priority information and E-UTRAN information. (Here, the term “3G” refers to information related to UTRAN.) For additional details, see chapter 10.5.2.33b of 3GPP TS 44.018. The number of SI 2quater message instances required to form a single coherent message set can thus be rather large, and it is not unlikely that as many as six instances are needed. For example, six instances of SI 2quater transmitted on the BCCH may consist of the following:                1st instance (SI 2quater_INDEX=0000)—3G Meas. Params Descr., GPRS—3G Meas. Params Descr., 3G Add. Meas. Params Descr., 3G Add. Meas. Params Descr. 2.        Next 3 instances (SI 2quater_INDEX=0001, 0010 and 0011)—3G Neighbour Cell Descr.        5th instance (SI 2quater_INDEX=0100)—3G Priority Params Descr., Serving Cell Priority Params Descr.        6th instance (SI 2quater_INDEX=0101)—E-UTRAN Params Descr.This example is illustrated in FIG. 2.        
Cell reselection from one GSM cell to another GSM cell or from a GSM cell to a WCDMA or LTE cell can be based on a method called Priority Based Cell Reselection, if priorities and thresholds are provided by the network and if the mobile station supports Priority Based Cell Reselection. Chapter 6.6.6 of 3GPP TS 45.008, v 10.1.0 (May 2011) provides further details of inter-RAT cell reselection based on priority information. With this technique, a mobile receives system information messages in the serving GSM cell that provide the mobile station with information about the priorities to be associated with the different RATs. While the GSM RAT has a single priority for the RAT itself (i.e., for all GSM cells), priority is provided on a per-frequency basis for UTRAN and E-UTRAN.
The specifications for Priority Based Cell Reselection require that system information never indicates the same priority for different RATs. However, the same priority may be used (but is not necessarily used) for frequencies within the same RAT. A range of eight different priorities (value range 0-7) are specified, in which 7 is the highest priority. Details of this scheme may be seen in chapter 9 of 3GPP TS 45.008.
According to the present specifications, in order to be able to select a new cell in another RAT the mobile must first retrieve all of the priority information available for all RATs and frequencies. In other words, a mobile station in idle mode must successfully read a complete coherent set of SI 2quater messages before it can make an inter-RAT reselection decision, unless one of the indicated RATs has been given the highest possible priority value (i.e., priority value 7) for all of its corresponding frequencies.